1. Field of the Invention
This invention generally relates to stranded cable manufacturing and more particularly to the manufacturing process for producing compressed concentric unilay stranded conductors with high speed single or double twist machinery, and cables and conductors produced thereby.
2. Description of the Prior Art
Compressed stranded cable conductors are well known in the art. Examples are disclosed in U.S. Pat. Nos. 4,473,995, 3,383,704 and 3,444,684. Such cables are preferred over uncompressed cables or compacted cables for several reasons. Compressed conductors typically have a nominal fill factor from about 81% to 84% Fill factor is defined as the ratio of the total cross-section of the wires in relation to the area of the circle that envelops the strand.
Uncompressed cables require the maximum amount of insulation because the cable diameter is not reduced and because interstitial valleys or grooves between the outer strands are filled with insulation material. Typical fill factors for these conductors are about 76%.
On the other hand, compact conductors, although eliminating the above-mentioned drawbacks, might have physical properties that are not desirable for specific applications. Typical fill factors for these constructions range from 91% to 94%.
Multiwire compressed conductor strands are made in different configurations and by many different methods. Each method and configuration has advantages and disadvantages. One approach is to form the strand with a central wire surrounded by one or more helically layered wires. The strand is made by twisting the wires of each layer about the central wire with a wire twisting machine. A true concentric strand is one example of a strand made by this method. Each layer of a true concentric strand has a reverse lay and an increased length of lay with respect to the preceeding layer. In case of a 19-wire conductor strand, two passes might be required through a wire twisting machine to make the strand.
One example of a known strand involves one pass for a 6-wire layer having, for example, a Right Hand lay over the central wire and a second pass for a 12-wire layer having a Left Hand lay over the first six wire layer. The strand can also be made in one pass with machines having cages rotating in opposite directions applying both layers at the same time, but the productivity of such machines is very low.
A unilay conductor is a second example of a conductor strand having helically laid layers disposed about the central wire. Each layer of a unilay strand has the same direction of lay and the same length of lay. Because each layer has the same lay length and same direction, the strand may be made in a single pass. As a result, productivity increases.
Unilay strands are used in a variety of configurations and commonly for sizes up to and including 240 sq. mm.
These strands can be manufactured either on a Single Twist machine or a Double Twist machine. The Single Twist machine has advantages over the Double Twist machine since strands made on such machines are generally more uniform than those used on Double Twist machines. This occurs because of the difficulty in a double twist machine of controlling the tension of the wire entering the closing die and because of the second twist that is applied to the wires after the cable has already been subjected to the first twist.
However, Double Twist machines have the advantage of higher productivity than Single Twist machines because, by its configuration, a Double Twist machine imparts two twists for each revolution of the flyer. Moreover, because of differences in construction, Double Twist machines can easily operate at higher rotational speeds than single twist machines.
As a result, the output of Double Twist machines is often more than three times the output of Single Twist machines for a similar strand.
Referring to FIG. 1, one of the most commonly used unilay conductors is a conductor S.sub.1 formed with 19 wires of the same diameter D. In such a strand, the six wires 4 of the inner layer L.sub.1 and the twelve wires 6 of the outer layer L.sub.2 are twisted about the central core wire 2 in the same way and in a concentric pattern. Normally a hexagonal pattern (dash outline H) is formed, and not the desired round configuration C. This hexagonal configuration presents many basic problems because the circumscribing circle C creates six voids V. These voids are filled with insulation requiring more insulation for a minimum insulation thickness as compared with a true concentric strand.
Experience has also shown that the wires at the corners tend to change position and to back up during extrusion.
As a result of this concern, engineers in the conductor wire industry have been seeking to develop conductor strands which maintain a circular cross-section and increase the uniformity of the conductor section.
One approach is to try to position the outer twelve conductors in such a way as to have each two wires 6a, 6b at the second layer L.sub.2 perched on the surface of one of the six wires 4 of the first layer L.sub.1. Such conductor S.sub.2, shown in FIG. 2, is sometimes referred to as having a "smooth body" construction which avoids the problem mentioned above in connection with the conductor 2 in FIG. 1.
However, the "smooth body" construction is not stable and cannot be easily achieved on a commercial basis without considerably reducing the lays and, therefore, the productivity of the machines. Furthermore, any variation in wire diameter or tension in the wires can cause the conductor strand to change into the hexagonal configuration shown in FIG. 1 which represents the stable, low energy construction.
Another attempt to solve the problem has been to make a composite strand S.sub.3 in accordance with U.S. Pat. No. 4,471,161 and shown in FIG. 3. This last construction has the advantage of being stable, but the disadvantage of requiring wires 6c, 6d with different diameters D.sub.1, D.sub.2 in the second layer L.sub.2. However, in order to maintain a circular outer cross-section, the diameters D.sub.1, D.sub.2 which must be selected result in gaps or grooves G between the wires into which insulation can penetrate. A variation on this idea is represented in FIG. 4 where the 7-wire core (1+6) is compressed, such compression allowing the smaller diameter wires 6d to move radially inwardly to a degree which substantially eliminates the tangential gaps in the 12-wire layer L.sub.2.
Another solution has been to use a combination of formed or shaped and round elements or wires to assure that the desired fill factor is realized with a stable strand design minimizing the outer gap area and optimizing the use of the insulating material. One example of such a strand uses a combination of 7 "T" shaped elements with 12 round elements providing a stable strand design. Such constructions are shown in publication No. 211091 published by Ceeco Machinery Manufacturing Limited, at page 537-7. In this construction, the outer 12 elements or wires are in contact with each other thereby minimizing the grooves or spaces and the fill factor is approximately 84%. In such a configuration, the outside wires abut against the flat surfaces of the inner layer and have no tendency to collapse into the minimal spaces or grooves therein. A modification of the aforementioned strand involves various degrees of compression of the round wires with the result that the range of fill factors can be increased from approximately 84-91%. Because the inner layer of the 7 conductors is also compacted in the inner layer elements produce a substantially cylindrical outer surface with interstitial grooves minimized or substantially eliminated. While this eliminates the aforementioned problem of the outer layer collapsing into the grooves of the inner layer, such cables have fill factors that are too high for many applications.